As compounds having a protein tyrosine phosphatase inhibitory action, the following compounds are known.    1) WO99/46244 discloses a compound of the formula: wherein    A is together with a double bond to represent furanyl, thiophenyl, pyrrolyl, oxazolyl, thiazolyl, imidazolyl, pyrazolyl, isoxazolyl, isothiazolyl, 1,2,3-oxadiazolyl, furazanyl or 1,2,3-triazolyl;    R1 is hydrogen, COR5 and the like;    R2 is COR5 and the like;    R3, R16 and R17 are each independently hydrogen and the like;    R4 is hydrogen, hydroxy and the like;    R5 is hydroxy and the like.    2) WO99/58520 discloses a compound of the formula: wherein     B is a carbon atom or a nitrogen atom;    D is an oxygen atom, a sulfur atom or a nitrogen atom;    E is a carbon atom or a nitrogen atom;    X is a carbon atom, a nitrogen atom, an oxygen atom or a sulfur atom;    Y is a bond, methylene, C(O) or CH(OH);    Z is CH═CH, a nitrogen atom, an oxygen atom or a sulfur atom;    R1 is a C1-12 alkyl; a C6-10 aryl; a C7-15 aralkyl; a halogen; trifluoromethyl; a C1-6 alkoxy; Het-alkyl (wherein the alkyl group includes 1 to 6 carbon atom(s)); a C6-10 aryl mono, di- or tri-substituted with trifluoromethyl, a C1-6 alkyl or a C1-6 alkoxy;    Het is     R7 is C1-3 alkylene;    G is an oxygen atom, a sulfur atom or a nitrogen atom;    R2 is a hydrogen atom, a C1-6 alkyl, a C1-6 alkoxy, a halogen or trifluoromethyl;    R3 and R4 are each independently a hydrogen atom; halogen; a C1-6 alkyl; a C6-10 aryl; a halogen; trifluoromethyl; a C1-6 alkoxy; a C6-10 aryl which is mono-, di- or tri-substituted with trifluoromethyl, a C1-6 alkyl or a C1-6 alkoxy; nitro; alkylsulfamide; arylsulfamide; a C3-8 cycloalkyl; or 5 to 7 membered heterocycle having 1 to 3 heteroatom(s) selected from an oxygen atom, a nitrogen atom and a sulfur atom;    R5 is a hydrogen atom; a C1-6alkyl; a C6-10 aryl mono-, di- or tri-substituted with trifluoromethyl, a halogen, a C1-6 alkyl, a C7-15 aralkyl or a heteroaryl; and    R6 is a hydrogen atom, —OR5 or —OCOR5;    provided that when R1 is a halogen, Y is a bond.
Moreover, pyrrole compounds of the following formula are known.    3) Farmaco, Ed. Sci (1984), 39 (9), p765 discloses a compound of the formula: 
4) Yakugaku Zasshi (1973), 93 (5), p584 discloses a compound of the formula: 
However, none of the above-mentioned pyrrole compounds is reported to be a protein tyrosine phosphatase inhibitor or a prophylactic or therapeutic agent of diabetes.
Therefore, there is a strong demand for development of a pyrrole compound which has protein tyrosine phosphatase inhibitory action and useful as a medicament such as a prophylactic or therapeutic agent for diabetes and the like.